The present invention relates to a signal distributing and muting system for use in a multiple channel frequency modulation stereo receiver. More particularly, the invention pertains to a composite signal and bias potential distributing arrangement for switching the receiver's demodulator circuitry between operative and quiescent conditions in response to a control signal while simultaneously muting the receiver during such switchover.
Present-day broadcast FM stereo features the transmission of a two-channel coherent stereo signal, the demodulation function of which may be represented as follows: EQU M'(t) = K'(L+R) = K"(L-R)sin .omega..sub.s t, (1)
where L represents a left side audio signal, R represents a right side audio signal, .omega..sub.s is the radian frequency of a suppressed carrier amplitude modulated subcarrier signal, t is time and K' and K" are constants. A two-channel stereo receiver responds to a stereo broadcast by demodulating the sum and difference audio terms and then matrixes those two terms in order to yield the fundamental left and right audio signals L and R. This same receiver responds to a monaural FM broadcast by reproducing the same monaural audio signal in both of its output channels.
On the other hand, a monaural FM receiver responds to a two-channel broadcast stereo signal by deriving only the sum term (L+R) represented in equation (1) to reproduce an audio signal representative of the monaural program. The two-channel signal thus is fully compatible with the monaural signal so that a receiver properly designed for one will also receive the other. Further detailed discussion of the foregoing two-channel transmission system and exemplary disclosures of transmitters and receivers for use therewith will be found in the following U.S. Pat. Nos.: 3,257,511--Adler et al.; 3,257,512--Eilers; 3,129,288--DeVries; and 3,151,218--Dias et al., all of which patents are assigned to the same assignee as the present invention.
In the last few years, interest has been evidenced in recording systems wherein a four-channel, or quadraphonic, stereo signal is recorded on magentic tape. The four different audio signals represent sources respectively located at the left front, right front, left rear and right rear of an originating point. By using four similarly located pick-ups to effect a four-channel recording and then, subsequently, utilizing a playback arrangement having four separate loud speakers similarly distributed around a listening point, a four-channel reproduction is obtained.
The advent of four-channel stereo recording and reproduction has naturally led to consideration of the desirability of transmitting and receiving four-channel stereo signals by radio. Because two-channel stereo is now being broadcast by many FM transmitting stations, attention has been directed particularly to the possibility of utilizing broadcast stations in that category of service for the transmission of four-channel stereo in addition to, or instead of, the transmission of two-channel stereo or monaural signals. To accomplish such a transmission a complex composite baseband signal must be developed in order to accommodate the additional signal components necessary to convey four separate channels of information. At the same time, it is desirable that any four-channel approach be fully compatible with present two-channel stereo and monaural, so that receiver obsolescence is avoided.
It is also desirable from the standpoint of broadcast station economics, that a commercial four-channel stereo system include provision for an SCA (Subsidiary Communications Authorization) channel. The above mentioned U.S. Pat. No. 3,944,747 of Eilers, as well as U.S. Pat. No. 3,902,018 of Fockens, describe four-channel compatible FM stereo systems capable of accommodating SCA.
In a four-channel stereo system of the type herein contemplated (and more fully described below), the composite baseband signal comprises a main audio signal and three modulated subcarriers. This composite baseband signal, in turn, is employed to frequency modulate an RF carrier. Subsequent to initial detection in the receiver, the baseband signal is demodulated by three differential amplifier type demodulators; one of which detects the sum component and the .+-. Y difference signals, another detects the .+-. X difference signals while the third detects the .+-. U diagonal difference signals.
However, when a four-channel transmission is interrupted or terminated, the X and U demodulators become non-functional, thereby precipitating a muting problem. In the absence of any muting provision, audible noise is contributed to the audio output reproducers by the non-functional demodulators. The prior art resolves this problem by resort to circuitry that decouples the switching subcarrier signal from the non-functional demodulators. However, while a measure of silent muting is now achieved, the composite baseband signal is still translated by the non-functional demodulators which occasions a deterioration in channel separation on two-channel stereo programs.